Well, don't know the mechanisms in the brain, but if its the normal homeostatic resting point of that brains default, then it wouldn't run out. It's kinda like with body weight, and one person has a lower default homeostatic bodyfat level, to the person with a higher one, they will be thinking "but how do they manage the constant hunger associated with being thin?". The answer is they don't, their body naturally operates at that range.

With the vast variation in biology in people, expect variation in brains, and variation in behaviours.

Other alternative explanations would be that they don't have those variations, they just force themselves to operate in that way. Or both.

I have recently had to operate behaviourally over the last few months in a different way entirely to how my biology is screaming at me (if you are dumped, love that person, have an anxious attachment style, feel the breaup was avoidable, that the relationship could have been amazing and saw them as a lifelong partner, that the person wants desperately to be friends, and you are desperately lonely and friendless, and that they live 5 minutes drive from you... voluntarily going no contact is basically excruciating, it feels like death. But I did it, because it serves my interests to do so.) It has therefore occured to me that people who achieve more, or are more outgoing, or whatever, may in large part simply be forcing themselves. That this capacity to force is somewhat biological isn't in doubt.. but it's worth considering.

Any studies that show elevated dopamine, or whatever, aren't actually showing a causal relationship, they are correlating that kind of behaviour with those biologies. That doesn't necessitate the brain is set like that.

It's probably more the former, though .